Make ImportFrom level just a u32 (#11170)

crates/ruff_python_semantic/src/star_import.rs

@@ -1,7 +1,7 @@
 #[derive(Debug, Clone)]
 pub struct StarImport<'a> {
-    /// The level of the import. `None` or `Some(0)` indicate an absolute import.
-    pub level: Option<u32>,
+    /// The level of the import. `0` indicates an absolute import.
+    pub level: u32,
     /// The module being imported. `None` indicates a wildcard import.
     pub module: Option<&'a str>,
 }